The main goal of this research is to characterize patient-specific respiration-induced tumor and surrogate motion to evaluate the accuracy and effectiveness of the surrogate-based motion management strategies currently used in clinics. Specifically, the investigators hypothesize that dynamic MRI obtained over a temporal duration consistent with radiotherapy treatments will provide spatio-temporal information of both the tumor and surrogate, and therefore can serve as a means to assess the quality of the tumor motion tracking with the surrogate. To test their hypothesis, the investigators specifically propose to 1) track and characterize the tumor and surrogate motion with 4D-MRI and 2) evaluate surrogate-based motion tracking in a cohort of patients with thoracic tumors.